DEFENSE REACTION IN THE POND SNAIL PLANORBIS-CORNEUS .3. RESPONSE TO INPUT FROM STATOCYSTS

1. In the intact pond snail Planorbis corneus, a rapid tilt in any plane evoked a defense reaction consisting of a fast movement of the shell towards the head, shortening of the foot, inhibition of locomotion and of rhythmical feeding movements. This reaction was similar to the first phase of the general defense reaction of Planorbis to cutaneous stimulation. 2. A method has been developed for inclination of the isolated CNS in space (up to 900) and simultaneous intracellular recordings from different neurons. 3. The statocyst receptor cells (SRCs) responded both phasically and tonically to the tilt. The SRCs differ in their spatial zones of sensitivity. 4. Essential manifestations of the defense reaction to the input from statocysts could be observed in the in vitro preparation of the CNS isolated with statocysts. Both tilting of the CNS and electrical stimulation of individual SRCs elicited an excitatory response in numerous neurons from different ganglia, including motor neurons (MNs) of the columellar muscle. This response was of ''all-or-none'' nature, and could be evoked by electrical stimulation of any SRC. The response was followed by a long( 10-20 s) period of refractoriness. 5. Activation of SRCs resulted also in excitation of the giant dopaminergic cell in the left pedal ganglion (related to the control of respiration), in inhibition of the feeding rhythm generator, and in inhibition of the pedal neurons responsible for activation of the ciliary locomotor system. 6. Combined stimulation of two inputs able to evoke a defense reaction, i.e., those from the statocyst and from cutaneous nerve, revealed a strong interdependence of their central effects. A defense response to a given input could not be evoked during the refractory period following the response to another input. This interdependence is most likely due to the fact that both inputs converge on the same interneuron (DRN1) in the right pleural ganglion. The ''all-or-none'' character of the statocyst-driven defense reaction, in contrast to the graded response to cutaneous nerve stimulation, could be explained if the input from statocysts activated only one DRN1.